This invention relates to wideband code division multiple access (WCDMA) for a communication system and more particularly to turbo-coupled multi-code multiplex data transmission for WCDMA signals.
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication. These WCDMA systems are coherent communication systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into sixteen equal time slots of 0.625 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 32 KSPS, for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. Each symbol includes I and Q bits corresponding to real and imaginary parts of a QPSK signal. The symbols are subdivided or spread into smaller signal units called chips. Each bit is typically spread over 16 to 256 chips according to a predetermined spreading rate. A pseudorandom noise (PN) generator generates each group of spreading chips. The initial state of this PN generator determines the spreading code of the chips. The chip transmission time (TC), therefore, is equal to the symbol time rate (T) divided by the number of chips (N) that spread the symbol.
Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
Referring to FIG. 4, this spreading of data signals over a wide bandwidth is accomplished by encoding a data sequence Dk on lead 100, for example, by a recursive systematic convolutional (RSC) encoder 400. The encoded data sequence on lead 402 is modulated by a CDMA spreading code by circuit 404. This M-sequence or Gold code is typically produced by a linear feedback shift register (LFSR) circuit such as the forward link LFSR of FIG. 3. The first LFSR 300 circuit is preferably initialized to a characteristic state of the base station. The second LFSR 310 is preferably initialized to an all-one state. The sequence of each LFSR circuit is typically defined by an LFSR polynomial. This polynomial is characterized by the number of shift register bits and the location of feedback taps such as feedback tap 304 of LFSR 300.
Previous studies have investigated turbo-code systems for improved transmission power in CDMA systems. In their paper, Claude Berrou et al., Near Optimum Error Correcting Coding and Decoding: Turbo-Codes, IEEE Trans. on Commun., vol. 44, no. 10, October 1996, at 1261, compare recursive systematic convolutional (RSC) codes to nonsystematic convolutional (NSC) codes. Referring to FIG. 10, there is an exemplary NSC encoder of the prior art. This NSC encoder receives an input bit X0 on lead 1000 and generates three output bits Y0, Y1 and Y2 on leads 1030, 1031 and 1032, respectively, for an R=1/3 transmission rate and constraint a length of 9. Series-connected register stages 1010 through 1017 sequentially shift input bit X0 through each stage in response to a clock signal (not shown). Input signal X0 and output signals from the eight register stages, therefore, influence output bits Y0, Y1 and Y2 for 9 clock periods. This finite period of influence permits use of a maximum likelihood finite-state decoder such as the Viterbi decoder.
By way of comparison, FIG. 11 is an exemplary RSC encoder of the prior art. The RSC encoder receives an input bit Xk on lead 1100 and produces the systematic output bit Xk together with parity bit Yk on lead 1126. This RSC encoder includes a feedback shift register with series-connected stages 1110 through 1113, which continually feed back the input signal via sum circuits 1130, 1132, 1134 and 1120. Thus, input bit Xk influences the generation of parity bit Yk for the entire encoding period. This expanded influence results in improved performance approaching the Shannon theoretical limit when used with a maximum likelihood Viterbi decoder.
Berrou et al. describe an exemplary RSC encoder having a transmission rate R=1/2 as in FIG. 1. This exemplary encoder 102 receives a data sequence Dk on lead 100. The RSC encoder 102 produces a coded data bit Xk and a redundant or parity bit Yk on lead 104. Berrou et al. further disclose a turbo-code RSC encoder (FIG. 2) including a first RSC encoder 200 and a second RSC encoder 210. The turbo-code interleaver circuit 206 typically comprises a matrix that receives data in row order and produces the data in column order to achieve maximum scattering of data and maximum disorder of the interleaved data. Berrou et al. state that the data may be completely transmitted for a rate R=1/3 encoding or punctured for higher rates. Id. at 1263. This punctured data transmission is accomplished by alternately multiplexing the output from RSC encoders 200 and 210 onto lead 214 with multiplex circuit 204. Thus the order of transmitted data is given by equations [1-2].
Xk=Xnxe2x86x92Xn+1xe2x86x92Xn+2xe2x86x92Xn+3 . . .xe2x80x83xe2x80x83[1]
Yk=Yn2xe2x86x92Yn+11xe2x86x92Yn+22xe2x86x92Yn+31 . . .xe2x80x83xe2x80x83[2]
Although the RSC turbo-code achieves efficiency near the Shannon limit, it must rely on puncturing to accomplish this end. This use of puncturing, however, fails to use an appreciable amount of available information.
The foregoing problems are resolved by a circuit designed with an encoder circuit coupled to receive a data sequence. The encoder circuit produces a first encoded data sequence and a second encoded data sequence from the data sequence. A first spreading circuit is coupled to receive the data sequence and the first encoded data sequence. The first spreading circuit produces a first modulated data sequence in response to a first code. A second spreading circuit is coupled to receive the data sequence and the second encoded data sequence. The second spreading circuit produces a second modulated data sequence in response to a second code.
The present invention improves transmission power by turbo-coupled multi-code multiplexing. No increase in coding rate is required. Communication is improved by controlling transmission power of redundant information without puncturing.